An electrochemical accumulator has a nominal voltage of the order of a few volts, to be more precise 3.3 V for Li-ion batteries based of iron phosphate and 4.2 V for a Li-ion technology based on cobalt oxide. If this voltage is too low relative to the requirements of the systems to be supplied with power, a plurality of accumulators are connected in series. It is equally possible to dispose one or more accumulators in parallel with each series-connected accumulator in order to increase the available capacity and to provide a higher current and a higher power. The parallel-connected accumulators then form what is referred to herein as a stage. A stage is constituted of at the minimum one accumulator. The stages are connected in series to reach the required voltage level. The association of accumulators is called an accumulator battery.
Charging and discharging an accumulator is reflected in an increase or a decrease, respectively, in the voltage at its terminals. An accumulator is considered charged or discharged when it reaches a voltage level defined by the electrochemical process. In a circuit utilizing a plurality of accumulator stages, the current in the stages is the same. The level of charge or discharge of the stages thus depends on the intrinsic characteristics of the accumulators, namely the intrinsic capacitance and the series and parallel parasitic internal resistances of the electrolyte or of contact between the electrodes and the electrolyte. Voltage differences between the stages are then possible because of fabrication disparities and ageing.
For a Li-ion technology accumulator, too high or too low a voltage, called the threshold voltage, may damage or destroy the accumulator. For example, overcharging a Li-ion accumulator based on cobalt oxide may lead to thermal runaway and start a fire. For a Li-ion accumulator based on iron phosphate, overcharging is reflected in decomposition of the electrolyte that decreases its service life or can damage the accumulator. Too great a discharge, for example leading to a voltage less than 2 V, leads primarily to oxidation of the current collector of the negative electrode if it is made of copper and thus to damage to the accumulator. Consequently, surveillance of the voltages at the terminals of each accumulator stage is obligatory during charging and discharging for reasons of safety and reliability. A so-called surveillance device in parallel with each stage enables this function to be provided.
The surveillance device has the function of tracking the state of charge and discharge of each accumulator stage and transmitting the information to the control circuit in order to stop charging or discharging the battery if a stage has reached its threshold voltage. However, in the case of a battery with a plurality of accumulator stages disposed in series, if charging is stopped when the stage with the highest charge reaches its threshold voltage, the other stages may not be fully charged. Conversely, if discharge is stopped when the most discharged stage reaches its threshold voltage, the other stages may not be totally discharged. The charge in each accumulator stage is thus not used optimally, which represents a major problem in transport and onboard type applications having severe autonomy constraints. To alleviate this problem, the surveillance device is generally associated with an equalization device.
The equalization device has the function of optimizing the charge of the battery and therefore its autonomy by bringing the accumulator stages connected in series to an identical state of charge and/or discharge. There exist two categories of equalization devices, so-called energy dissipation equalization devices and so-called energy transfer equalization devices.
In the case of energy dissipation equalization devices, the voltage at the terminals of the stages is rendered uniform by bypassing the charging current of one or more stages when the threshold voltage has been reached and dissipating the energy in a resistor. Alternatively, the voltage at the terminals of the stages is rendered uniform by discharging one or more stages where the threshold voltage has been reached. However, such energy dissipation equalization devices have the major drawback of consuming more energy than is necessary to charge the battery. This circuit makes it obligatory to discharge a plurality of accumulators or to divert the charging current of a plurality of accumulators so that the last accumulator or accumulators with a slightly lower charge finish(es) charging. The energy dissipated can therefore be very much greater than the charge or charges to be completed. Moreover, the excess energy is dissipated as heat, which is not compatible with integration constraints in transport and onboard type applications, and the service life of the accumulators falls sharply if the temperature is raised.
Energy transfer equalization devices exchange energy between the accumulator battery or an auxiliary power network and the accumulator stages.
There is known from the U.S. Pat. No. 5,659,237, for example, a device enabling transfer of energy from the auxiliary network to the stages by a “flyback” structure with a plurality of outputs using a coupled inductor as a storage element. The latter element is a specialized component because it is dedicated to this application. The cost of such a component is prohibitive for the function to be implemented.
There is known from the patent CN1905259 a device enabling transfer of energy from the stages to the battery and that for its part uses an inductor for each accumulator as a storage element. However, this device does not opt for optimized energy transfer for equalization of the batteries in transport and onboard type applications. The end of charging of a battery is indeed determined by the last stage to reach the threshold voltage. To terminate the charging of the battery, energy is taken from one or more stages and returned to the group of stages. If one or more accumulator stage(s) is or are slightly less charged, energy is then not transferred with priority to the latter stage(s) that require(s) it but also to the stage(s) from which the energy is taken. Equalization thus necessitates taking energy from all stages at the end of charging in order to prevent charging them to too high a voltage. Equalization is thus effected with high losses because of the large number of converters operating. Moreover, accumulators for which charging has already ended have alternating or direct current components of no utility pass through them.
An object of the invention is therefore to propose an improved equalization device that does not have these drawbacks of the prior art.